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Knotweed damage

What Is Japanese Knotweed?

Japanese Knotweed is the most invasive species in Britain and can have costly legal implications for those who have Knotweed on their commercial or residential property.

Where is Japanese Knotweed in the UK?

Knotweed dispersal
Image credit: NBN Atlas

Why is Japanese Knotweed a Problem?

Japanese knotwed damage

Japanese knotweed is an invasive plant that can spread aggressively outside its natural range in east Asia. It was introduced to the UK in the 1800s and has spread across the country causing damage to built structures and wild habitats.

Japanese knotweed exhibits vigorous growth through the soil from a fibrous rhizome (creeping root system) that can cause damage to areas of hard standing, foundations, drains, services, walls and other built structures by growing through cracks and openings between them. Soil can be contaminated with reproductive knotweed material up to a depth of 3m and a radius of over 7m.

The Environment Agency recommends the removal and/or treatment of all soil within these extents.

If it is not dealt with before a site is redeveloped, or if groundworks take place for another reason, Japanese knotweed contaminated material can very easily be spread across or off a site, resulting in new areas of knotweed growth and, potentially, criminal prosecution or litigation from third parties. This is because from only a small amount (less than a centimetre) of knotweed root (rhizome), regeneration can occur forming a new, viable plant.

What does Japanese knotweed do to a house?

If allowed to accumulate and grow over many years, the roots and rhizomes of Japanese knotweed can grow through gaps in built structures and floors, prising them apart and causing damage as the rhizomes expand and develop into large crowns. Damage can also be caused to drains and sewers as Japanese knotweed rhizomes can grow many metres through them looking for gaps and cracks to grow out of.

Is knotweed harmful to humans?

Japanese knotweed is not poisonous or otherwise harmful to humans. In fact, many parts of the plants can be eaten or have been used in traditional or alternative therapies and medicines. The only real risk of harm likely to be caused by Japanese knotweed is if its accumulated growth causes a heavily built structure like a wall to fall over.

How do you get rid of Japanese knotweed?

It is not easy to get rid of Japanese knotweed. Its roots and rhizomes can grow very deep into the ground and spread several metres laterally beyond the stems. This presents an iceberg analogy of there sometimes being a much larger amount of biomass below ground compared to the visible stems above ground.

This means that herbicides applied to the leaves often don’t reach the roots, making herbicide control difficult and potentially taking many years. Another method to get rid of Japanese knotweed is to completely dig it out. However, owing to the often large size and extent of the rhizomes, this can require great effort and result in large volumes of infested soil, which can be very expensive to dispose of.

If you are looking to identify Japanese knotweed, we have a range of resources as well as a dedicated identification service that can help; if you prefer, send us a picture for your free assessment. If you have received confirmation that you have Japanese knotweed, we offer a Japanese knotweed surveys for domestic and commercial premises.

If you think that you have Japanese Knotweed on your property or construction site, get in touch with our team of experts for your FREE consultation.

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